How to Install an Interlock Kit and Power Inlet Box
Why this matters
An interlock kit and an inlet box are the cheapest legal way to put a portable generator on a house panel, and the most common way a shop ends up on the wrong side of an inspector. The job looks small: a plate, a breaker, a box on the wall. The failures are not small. A kit that is not listed for that panel gets red-tagged, a backfed breaker with no hold-down can be pulled out live, and an inlet put in the wrong spot drags the generator up to a window. Get the sequence right once and the job is the same every time.
Who does this work, and what the plate does not do
Service and branch-circuit work goes to a licensed electrician wherever your state or locality licenses it, and almost every jurisdiction does. Most also want a permit and an inspection for this job, because it adds transfer equipment to an optional standby system under NEC Article 702 as adopted by your jurisdiction. If your shop's generator techs are not licensed for panel work, the tech's part is the site walk, the inlet location and the customer handoff; the electrician lands the conductors.
The interlock is mechanical, and that is all it is. It stops the main breaker and the generator breaker from being on at the same time. It does not start anything, shed anything or limit anything; the homeowner is the load manager. That idea is owned by a sibling card, so it gets one line here. See related: Interlocks and Why They Are Not Control.
Make the panel dead before anyone touches it
The main breaker does not make the panel dead. The line side of the main, its lugs and the service conductors above it stay energized from the utility whatever the handle says. Plan the job so nothing is done near those lugs, and where the work needs the panel fully dead, the utility pulls the meter by arrangement.
- Open the main breaker, then lock and tag it under 29 CFR 1910.333(b)(2), or 1926.417 where the job is construction work.
- Prove the load side dead with a tester proven on a known live source before and after the check, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024): bus to bus, each bus to neutral, each bus to ground.
- Treat the line side as live throughout. Insulated tools, and no hands above the main.
The restore step, with its own proof, closes the procedure.
The layout you are building
OUTSIDE WALL INSIDE, MAIN PANEL
+---------------------+
30A inlet box | MAIN 200A | GEN 30A|
L14-30 male pins | [ON ] | [OFF] |
X Y N G | interlock plate |
| | | | | slides: one or |
| | | +-- green 10 AWG --+-> ground bar |
| | +----- white 10 AWG --+-> neutral bar |
| +-------- black 10 AWG --+-> GEN breaker, pole2|
+----------- red 10 AWG ----+-> GEN breaker, pole1|
1/2 in conduit, 15 ft | hold-down screw on |
| GEN breaker |
25 ft 10/4 cord to the | branch breakers |
portable, set 20 ft out | below |
+---------------------+
The generator breaker sits in the top slots next to the main, in the exact position the kit's instructions name, because that position is what lets the sliding plate block one handle whenever the other is on.
Step 1: Prove the kit is listed for this panel
Read the panel's label inside the door: maker, model or catalog number, bus rating and the breaker types it accepts. Then read the interlock kit's instructions for the list of panels it is listed or evaluated for. The match has to be exact. A kit that "fits" a similar panel from the same maker is not the same as one listed for it, and inspectors check this first because it is the easiest finding to make.
Why first: everything else depends on it. If no listed kit exists for this panel, the answer changes to a small manual transfer switch or a replacement panel, and you want to know that before the inlet box is on the wall.
Also check the top slots. If they are occupied, those circuits have to move down, and if the panel is full the job may need tandem breakers (only where the panel's label permits them in those slots) or a subpanel. A panel with no room is a quote change, not a reason to put the generator breaker somewhere the plate cannot reach.
Step 2: Site the inlet with the generator's position in mind
The inlet location sets where the portable will run, so site the generator first and the inlet second. The portable runs at least 20 feet from the house with the exhaust pointed away from the building, and never in a garage, carport, crawlspace or under a porch with the door open. Pick a spot on open ground, away from windows, doors and fresh-air intakes, then put the inlet where a cord reaches it from there. See related: Exhaust Clearance and CO Safety Standard.
The common mistake is mounting the inlet beside the panel for the shortest conduit run, on a wall where the only flat ground within a cord length is the driveway against the garage door. The customer then runs the generator there. A 25-foot cord from a 20-foot setback leaves little slack, so the inlet usually lands on the wall facing the open yard.
Mount it above snow and splash, in the box's weather-rated orientation.
Step 3: Size the conductors to the inlet, not the generator
The inlet, the cord, the conductors and the generator breaker are all one rating, and the weakest piece sets it. Match the inlet to the generator's largest 240-volt receptacle: a 30-amp twist-lock generator gets a 30-amp inlet; a 50-amp receptacle gets a 50-amp inlet.
Conductors are sized from NEC Table 310.16 as adopted, at the temperature rating of the terminals, with the small-conductor limits of NEC 240.4(D) applied. For a 30-amp inlet that is 10 AWG copper, which carries 30 amps in the 60 C column and is capped at 30 amps by 240.4(D). For a 50-amp inlet, 6 AWG copper covers it in the 60 C column; 8 AWG copper works only where every termination is rated 75 C.
Pull four conductors: two hots, a neutral and an equipment ground. The neutral is needed because the house has 120-volt loads that will run off the generator. Pull a green conductor even in metal conduit; it removes the argument.
Step 4: Install the generator breaker and hold it down
Install a two-pole breaker of the type the panel label accepts, rated to match the inlet (30 amps here), in the slots the kit names. Land the red and black on its two poles, the white on the neutral bar and the green on the ground bar.
Then fasten it. A plug-in breaker that is backfed, meaning the supply comes in through what is normally its load terminals, must be secured by an additional fastener that takes more than a pull to release, per NEC 408.36 as adopted by your jurisdiction (408.36(D) in recent editions; check the current one). The kit or the panel maker supplies a hold-down. Without it, a breaker pulled off the bus while the generator runs has energized stabs in someone's hand.
Torque every terminal to the value on the breaker and the panel label with a calibrated torque screwdriver. Loose aluminum or copper on a backfed breaker shows up later as a burned stab.
Step 5: Fit the plate and test it dead
Mount the interlock plate per the kit's instructions, using its fasteners and the panel cover's own holes or the ones the instructions tell you to drill. Then test the mechanism with everything still dead and locked out:
- Main ON: the generator breaker handle must be physically blocked from ON.
- Slide the plate, main OFF: the generator breaker must go ON, and the main must now be blocked.
- Try to force each blocked handle by hand. If either moves, the plate is mispositioned or the breaker is in the wrong slot.
Why test dead: this is the one check that proves the plate works, and you want to find a gap with the bus dead, not with a generator running and a lineman upstream.
Step 6: Label and hand off
The panel gets a label at the generator breaker and the interlock, and the inlet gets a sign identifying it as a standby power inlet and the type of source, per NEC 702.7 as adopted (the exact wording and placement rules move between editions; follow the one your jurisdiction uses). Many inspectors also want the generator's neutral arrangement noted.
The neutral question goes on the handoff. With an interlock, the neutral is not switched, so the house panel's neutral-to-ground bond is the system bond. A portable with its own bond produces parallel neutral current on the ground path and nuisance GFCI trips. See related: Bonded Versus Floating Neutral on Portable Generators.
Restore and prove it, then run the generator
- Remove locks and tags, check no tools are in the panel, and replace the cover with the plate in place.
- Generator breaker OFF, main ON. Measure at a receptacle near the panel: about 120 volts hot to neutral, under a volt neutral to ground. That proves the utility side is back and the neutral was not disturbed.
- Set the portable at its sited spot, exhaust away. With it off and its output breaker open, connect the cord to the inlet and then to the generator, start it, and close its output breaker.
- Main OFF, slide the plate, generator breaker ON. With the cover on, turn on one branch breaker on each leg and read about 120 volts hot to neutral at a receptacle on each.
- Turn on a few branch breakers within the generator's rating and check a receptacle on each leg.
- Reverse: generator breaker OFF, plate back, main ON, confirm utility voltage again. Shut the generator down and disconnect the cord.
Worked example: a 30-amp portable on a 200-amp panel
The customer has a portable with a 7,500-watt running rating and an L14-30 receptacle. The panel is a 200-amp main breaker panel, 40 spaces, and the maker's interlock kit lists this exact catalog number. The top-right two slots hold a 20-amp single-pole bath circuit and a 15-amp lighting circuit; both move to open slots lower down.
The 30-amp rating on the receptacle sets the system: 30 amps at 240 volts is 7,200 VA, which is under the generator's 7,500-watt running rating, so the inlet, not the engine, is the limit. Inlet 30 amps, breaker 30 amps two-pole, four 10 AWG copper conductors in 1/2-inch conduit, 15 feet from the inlet to the panel.
Voltage drop at a full 30 amps, using about 1.24 ohms per 1,000 feet for 10 AWG copper:
- Cord, 25 feet, 50 feet round trip: 1.24 x 50 / 1,000 = 0.062 ohms, x 30 A = 1.86 volts.
- Building run, 15 feet, 30 feet round trip: 0.037 ohms, x 30 A = 1.12 volts.
- Total about 2.98 volts, 1.2 percent of 240.
That is fine, and it is why the shop does not upsize to 8 AWG here. Cross-check from the other direction: the whole loop is 80 feet of conductor, 1.24 x 80 / 1,000 is 0.099 ohms, and 0.099 x 30 is 2.98 volts, so the two figures agree.
The generator sits on the back lawn, 22 feet from the rear wall with the exhaust facing the fence, so the inlet goes on the back wall, not beside the panel on the garage side. The inside run gets longer by 6 feet, which is the right trade.
What would change it. A 50-amp generator receptacle means a 50-amp inlet, 6 AWG copper (or 8 AWG only with 75 C terminations throughout) and a 50-amp breaker. A panel with no listed kit means a manual transfer switch instead. And where the customer's real goal is to keep the heat pump and the well running without thinking about it, an interlock is the wrong product and the conversation moves to a standby with an automatic transfer switch.
References
- NFPA 70, National Electrical Code, as adopted by your jurisdiction: Article 702, 408.36, Table 310.16 and 240.4(D)
- NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333 and 1926.417
- The interlock kit maker's instructions and panel listing; the panel and breaker labels
- See related: Interlocks and Why They Are Not Control; Bonded Versus Floating Neutral on Portable Generators; Exhaust Clearance and CO Safety Standard